Along with photoresists and the like, certain optical media, including at least some silica-based optical fibers, can be modified by exposure to electromagnetic radiation in an appropriate spectral range. (Such radiation, typically ultraviolet radiation, is referred to below as "actinic" radiation.) That is, exposure of a photosensitive optical fiber (or other optical medium) to actinic radiation may cause the refractive index to change in the exposed portion of the medium. A periodic pattern can be imposed on the impinging radiation by, e.g., superimposing a pair of beams of substantially monochromatic radiation from, e.g., a laser, to create an interference pattern. If two beams of wavelength .lambda., intersect at an intersection angle .phi., the resulting interference pattern will have a period .LAMBDA. given by ##EQU1## When such a patterned radiation field impinges on a optical fiber or other optical waveguide having a core of the appropriate photosensitivity, a corresponding pattern is imposed on the core in the form of periodic (or quasiperiodic) fluctuations in the core refractive index. Such a pattern, which is often referred to as a "Bragg grating" or a "distributed Bragg reflector (DBR)" can behave as a spectrally selective reflector for electromagnetic radiation. Bragg gratings formed in this manner are particularly useful as end-reflectors in optical fiber lasers. These Bragg gratings are useful both because they are spectrally selective, and because they are readily incorporated in the same optical fiber that supports the active laser medium.
A technique for creating these Bragg gratings is described in U.S. Pat. No. 4,725,110, issued to W.H. Glenn, et al. on Feb. 16, 1988, and U.S. Pat. No. 4,807,950, issued to W.H. Glenn, et al. on Feb. 28, 1989. An optical fiber laser having a DBR-terminated cavity is described in G.A. Ball and W.W. Morey, "Continuously tunable single-mode erbium fiber laser", Optics Left. 17 (1992) 420-422.
Bragg gratings are useful as passive optical components for other applications besides end-reflectors in fiber lasers. For example, Bragg gratings are useful as spectral filters for wavelength-division multiplexing and other optical signal-processing applications. An optical filter which comprises a Bragg grating formed in an optical fiber is described in U.S. Pat. No. 5,007,705, issued to W.W. Morey, et al. on Apr. 16, 1991.
Similar techniques are useful for forming a grating pattern in a photosensitive medium such as a photoresist overlying a substrate. The substrate is lithographically processed after exposure and development of the resist.
For some applications, it is desirable to provide a Bragg grating that is quasiperiodic instead of periodic. That is, the period of the grating (i.e., the linear distance, along the propagation axis, between successive peaks or valleys of the refractive index profile) is not a constant, but instead changes in a predetermined fashion along the propagation axis. The most common quasiperiodic grating is one in which the period increases or decreases as a function, typically an approximately linear function, of position along the propagation axis. Such a grating is referred to as a "chirped" grating. Chirped gratings are useful, inter alia, for making broadband optical reflectors. An application of chirped gratings in optical fiber communication lasers is described in co-pending U.S. patent application Ser. No. 07/827,249, filed by R. Adar et al. on Jan. 29, 1992. An application of chirping to remove undesirable structure from grating reflectivity spectra is described in the co-pending U.S. patent application entitled "Method for Forming Distributed Bragg Reflectors in Optical Media", filed by V. Mizrahi et al.
In the conventional method for making chirped gratings (in photoresists), the interfering beams that impinge upon the photosensitive medium are not collimated. Instead, each is made to diverge at a predetermined divergence angle. As a consequence of the divergence of the beams, there lacks a single, well-defined angle of intersection between the beams. Instead, there is an effective angle of intersection that depends upon position (measured along the propagation axis of the photosensitive medium) within the interference pattern. As a result, a grating is formed that has a spatially dependent period. This method is described in X. Mai, et al., "Simple versatile method for fabricating guided-wave gratings", Appl. Optics, 24 (1985) 3155-3161.
This conventional method is disadvantageous because it cannot be used to make a grating in which the period has an arbitrary spatial dependence. Instead, this dependence can only take a form that is accessible by the method of diverging the beams.